In a conventional optical disc system, to sense the position of the laser beam in relation to the track on the disc, the main laser beam creates a reflection from the disc. The reflection is typically picked up by 4 photo-diode sensors. FIG. 1 is a conceptual diagram illustrating how such a photo-diode configuration is laid out in relation to the track direction. The outputs of the 4 photo-diodes (when the laser beam is focused on the disc) are shown as signals A, B, C and D, respectively.
To control the position of a main laser beam 20 on the disc 22, the position of the objective lens 24 of the Optical Pickup Unit (OPU) 25 related to the track 26a-26n on the optical disc 22 is controlled by a voice coil motor. The OPU 25 is a device configured to emit a laser beam and pick up the optical reflection of the laser beam. The OPU 25 converts such reflections to electrical signals. The position of the lens 24 and a housing 28 of the OPU 25 are controlled by a sled motor (not shown) which moves the whole OPU 25. FIG. 2 illustrates the motion of the OPU 25 and the lens 24.
To direct the laser beam 20 to a particular track of the optical disc 22, or to cross some of the tracks 26a-26n of the optical disc 22, the voice coil motor and the sled motor are controlled at the same time. The voice coil motor controls the position of the lens 24 in relation to the optical disc 22, while the sled motor controls the position of the lens 24 in relation to the lens housing 28. Various conventional approaches are used to control the position of the lens 24 in relation to the lens housing 28. One conventional control approach is based on the center error signal CE. Such a conventional approach has positive characteristics such as improving the balance of two sides of the signals A and D versus the signals B and C of the main beam when the laser beam 20 stays on track. Also, such an approach can control the position of the objective lens 24 inside the OPU 25 to avoid hitting the lens 24 against the lens housing 28.
While such an approach may be effective on certain discs, with certain other discs (e.g., from a different manufacturer, a different fabrication run, etc.), a different reflection will result in a different amplitude for the center error signal CE. The different amplitude will potentially disturb the center control loop and potentially degrade control quality. In the worst case, the different value can create instability of the control loop.
It would be desirable to provide a method and/or apparatus to ensure quality of the center control of a closed-loop system by calibrating the amplitude of the control error signal.